Television signal-translating system



Jan. 13, 19 4 2. A. V.'LOUGHREN TELEVISION S IGNAL-TRANSLATING SYSTEM Filed May 31, 1959 m mwe 1 I I I I I I I g V N 82333 8532. E c m m R m m w D N GE m. m a R 9 A m u m W... m um .2. I qw a WWW Bu:

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ATTORNEY Patented Jan. 13, 1942 r v TELEVISION sIGNAL-mANsLATm Y srs'ratr.

Arthur V. Loughrem Great Neck, N. Y., assignor to liiazeltine Corporation, a corporation 01' Delaware Application my 31, 1939, Serial No. 276,594

8 Claims. (01. ire-7.5)

This invention relates to television signaltranslating systems and more particularly to an arrangement for stabilizing a video-frequency television signal and controlling the' contrast ratio of the repeated signal in a single vacuumtube amplifier circuit. The invention is, therefore, especially directed to the provision of an improved background-illumination insertion and contrast-adjustingv system fortelevision receivers.

In accordance with present television practice, a transmitted signal comprises a carrier wave modulated during successive intervals, or trace periods, by video-frequency and unidirectional components representative of light variations in an image being transmitted and of its average background illumination, respectively. Between the trace periods, that is, during retrace intervals, the carrier has a' predetermined amplitude level referred to as the blanking level, corresponding to a given shade, usually black, and is modulated during a part of this retrace period to initiations of successive lines and fields in the scanning of the image.

At the receiver, a beam is so deflected as to scan and illuminate a target or screen in a series of fields of parallel lines. The synchronizing components of the received signal are separated from the other modulation-signal components and utilized to control the scanning apparatus of the receiver so asto synchronizeits operation with that of similar apparatus utilized at the transmitter in developing the signal. The intensity of the beam is controlled by the transmitted light-modulation components, thereby to reconstruct the image.

An important requirement in a properly op-- crating television receiver is accurate stabilizavtion of the modulation signal with respect to'a by synchronizing componentswhich correspond appear as the same shade in the reproduced that is, the contrast ratio of the reproduced image should be adjustable at the receiver in accordance with conditions of reception and the type of picture beingreceived.

To these ends, it is attempted in conventional television receiving systems so to control the syste that the modulation signal is stabilized with respect to a particulansignal level as applied to the signal-reproducing device and to provide alsoan arrangement for controlling the amplitude range of the signal input to the reproducing device. As a result, the signal level or amplitude corresponding to black or any predetermined shade value is efiectively held approximately fixed with respect to a particular point on the signal-input-brightness characteristic of the reproducing device of the system and the gain of amplifier circuits in the system is adjusted in order to control the contrast ratio. Certain arrangements of the prior art have provided a separate tube, usually a'diode, for stabilizing the video-frequency modulation signal and have provided gaincontrol in one or more additional amplifier tubes in order to control the contrast ratio. Other arrangements of the prior art have provided stabilization by means of peak rectification in the input circuit 'of a video-frequency am= plifler tube and have provided for contrast-ratio particular signal level, usually the peak level representing the synchronizing-signal components, as the signal is applied to the reproducing device, in order to ensure that the background-illumination component is present in the signal. Stabilization is also necessary for setting of the black level in the signal-reproducing device; that is, for optimumv operation, the system must be so controlled that a given signal level or amplitude corresponding to black, or any predecontrol by means of amplification control of a separate video-frequency amplifier. In prior art arrangements, it has not been possibleto provide both of these controls in the same stage of a "video-frequency amplifier for the reason that, in

termined shade value, is effectively held fixed v with respect to the signal input-brightness characteristic of the signal-reproducing device of the system, in order that signal components representing any other given shade will, at all tinies,

the prior art, the arrangement utilized to control the amplification of the video-frequency amplifier to control the contrast ratio of the re-.

stabilizing and contrast-ratio control circuitthe eye of an observer is responwhich is free from one or more of the abovementioned disadvantages of the arrangements of the prior art.

It is a further object of the invention to provide a television signal-translating system including a stabilization control and contrast-ratio control embodied in a single stage of amplification.

In accordance with the present invention, a television signal-translating system comprises a. signal-translating channel adapted to have applied thereto an unstabilized television-modulation signal having trace and retrace intervals and varying during trace intervals in accordance with video-frequency components having a wide range of frequencies and during retrace intervals in accordance with synchronizmg-signal components. A video-frequency amplifier stage including a single vacuum-tube having an input circuit is provided in the signahtranslating channel, together with means for adjusting the effective transconductance of the vacuum tube to control the gain of the amplifier over the frequency range of the video-frequency components of the signal, thereby to control the contrast ratio of the reproduced picture. Effective transconductance, as utilized in this specification, relates to the change in the output current of the amplifier for a given change in input voltage thereto. Means are also comprised within the above-mentioned amplifier stage substantially unresponsive to the gain control of the stage for stabilizing the translated modulation signal at the input circuit of the amplifier at a predetermined level, usually the peaks of the synchronizing signal components.

In accordance with the preferred embodiment of the invention, the means for adjusting the gain of the amplifier stage comprises means for adjusting degeneration in the stage, while the grid circuit of the tube acts as apeak rectifier stabilizer substantially independent of the circuit for providing a degenerative control, thus to provide stabilization which is substantially unresponsive to the gain control utilized for contrastratio control.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. Y

In the accompanying drawing, Fig. 1 is a circuit diagram, partly schematic, of a cathode-ray tube television=receiving system including circuits embodying the present invention, while Figs. 2 and.3 are graphs utilized to explain the operation of the circuit of Fig. 1.

Referring now more particularly to Fig. 1 of the drawing, the system there illustrated comprises a receiver of the superheterodyne type including an antenna system II, II, connected to a radio-frequency amplifier I2 to which are connected in cascade, in he order named, an oscillator-modulator ii, an intermediate-frequency amplifier H, a detector 15, a video-frequency amplifier l8, and a cathode-ray signal-reproducing device I1. The cathode-ray tube l comprises the usual envelope I8 containing, in the order named. a cathode II, a. signal input or control grid 2|. and other conventional elements (not shown) of a cathode-ray signal-reproducing tube. Line-frequency and field-frequency scanning coils fleeting the beam in two directions normal to each other.

A. line-frequency generator II and a field-frequency generator 32 are also provided, coupled to the output circuit of detector l5 for synchronization and connected to scanning coils and 21, respectively, of the cathode-ray tube in a conventional manner. Suitable means are included in the circuits of generators 3i and 32 for selecting the synchronizing-signal components from other modulation components and from each other in a conventional manner. The stages or units just described, with the exception of videofrequency amplifier Ii, presently to be described, may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly to the general operation of the system described above as a whole, television signals intercepted by the antenna circuit 90, ll

' are selected and amplified in the radio-frequency 2 and 21 are disposed about the tube for deamplifier I2 and supplied to the oscillator-modulator 13, wherein they are converted to intermediate-frequency signals which, in turn, are selectively amplified in the intermediate-frequency amplifier I4 and delivered to the detector I5. The modulation components of the signal are derived by the detector l5 and are supplied to the video-frequency amplifier lfi'wherein they are amplified and from which they are supplied to the control grid 2| of the cathode-ray tube H in a'conventional manner. The detected signal components are also supplied to the generators II and 32 to synchronize the operation thereof. Under proper operating conditions of the tube ll, an electron beam is emitted from the cathode I! and accelerated and focused on its fluorescent screen and its intensity is controlled by the control grid 2| in accordance with the video-frequency voltages and control voltages impressed thereon.

Saw-tooth current waves are generated in linefrequency and field-frequency generators 3| and 32, which are controlled by the detected synchronizing-signal components, and these waves are applied to the scanning elements 26, 21 of the cathode-ray tube I! to produce electric scanning fields, thereby to deflect the ray in two directions normal to each other so as to trace a rectilinear scanning pattern on the screen of the tube and reconstruct the transmitted picture.

Referring now more particularly to the portion of the system embodying the present invention, the video-frequency stage l6, included in the signal-translating channel of the receiver. comprises a single vacuum tube 35 having input electrodes coupled to detector l5 and output electrodes'coupled to the cathode-control grid circuit of signal-reproducing tube l1 through a load circuit including a resistor 36 through which a. unidirectional operating potential is applied to the anode of tube 35 from a suitable source indicated as +B. A potential divider including a series-connected resistor I! and resistor 38 having an adjustable tap is coupled across the source of anode supply of tube 3!, by-pass condenser 39 being connected between the tap on resistor 38 and ground and the tap on resistor 38 being connected directly to the cathode of tub [1 to provide a bias therefor. An unby-passed cathode resistor II is connected in the common signalinput and signal-output circuits of tube 35 and has an adjustable tap thereon for providing adjustable degeneration to control the gain of the stage of the video-frequency amplifier including of the reproduced picture. The input circuit of tube 35 is connected and proportioned to operate as a peak detector in order to stabilize the translated signals and, for this purpose, the grid o! tube 35 is coupled to detector It by way of a chronizing pulses and that this voltage is -im-,

pressed upon the input circuit of the stage of amplification including tube 55 through coupling condenser 45. This wave form illustrated corresponds, in general, to the video-frequency wave tom of conventional television practice with the direct-current component removed. Due to the action of the coupling condenser'dd, the unidisectional component of the signal supplied by detector 23 is lost, if it is present in the output signal derived from detector i5, and the signal tends to center itself about the zero axis and to appear, therefore, as illustrated by curve $2. In order for a signal of the wave form of that of curve 42 to be useful at the reproducing device l'i, it must be stabilized, as explained above. The circult constants of the input circuit of tube 35 are so proportioned that the grid condenser dd and grid leak M cause the tube 35 to operate as a peak grid rectifier, automatically varying the bias of the control grid of tube 35 which, in the absence of a signal, is provided with zero bias.

The signal is thus stabilized at the output circuit of tube 35 as shown by curve it; that is, the synchronizing peaks of modulation are established at a substantially fixed level as they appear in the output circuit of the tube, and the unidirectional background-illumination component is thereby reinserted in the signal as applied to the cathode-ray reproducing devic ll.

Furthermore, as explained above, since the eye of an observer is responsive primarily to the contrast or ratio between the illumination values rather than the absolute value of any given shade of illumination of an object, it is desirable to prcvide'an arrangement for adjusting the (3011-,

trast ratio of the signal reproduced by tube 87. in accordance with the type or television picture being viewed and other operating conditions. The contrast ratio oi the reproduced signal depends upon the ratio of the illumination corresponding to white to that of black. This is more clearly illustrated by the curves of Fig. 2 in which the signal-input-brightness characteristic of cathode-ray tube ii is represented by curve A. For a signal input in accordance with curve B and sistor it, as connected in the circuit of Fig. 1, is an unby-passed cathode resistor and is, therefore,eflective to provide degeneration in the stage of amplification. Thus, the contrast of the re- 1 produced signal maybe adjusted by adjusting I the tap on resistor 40 to control the amount of degeneration in the stage of amplification including'tube 35. Inasmuchas the load circuit comprising, resistor ll of the input or detector portion of tube 85 is separate from the degenerative circuit including resistor 80, the peak detector efiectively provided is unaffected by changes of value of resistor and the signal output of detector I5, as applied through coupling condenser 46 and representedv by curve 52, is stabilized, as illustrated in curve $3, on the peaks of the synchronizing pulses regardless of variations in, the adjustment of the tap on resistor 40. Stated, in another way, since the grid leak is returned from the control grid to the cathode of tube 35, the direct potential lead in the control grid cathode path will always be established at approximately zero volts for the most positive portion of the input signal independent of the setting of cathode resistor 50.

In summary, therefore, the television signaltranslating stage of vthe invention comprises a signal-translating channel, adapted to have applied thereto irom detector is through condenser M, an unstabilized television modulation signal having trace and retrace intervals'and varying during trace intervals in accordance with videofrequency' components having a wide range of frequencies and during retrace intervals in ac- The video-frequency stage it in this channel includes a single vacuum-tube amplifier 35 having an input circuit and including the adjustable unby-passed degenerative resistor d0, common to the input and output circuits of tube 35, which provides means for adjusting the effective transconductance of the vacuum tube 35 to control the gain of the amplifier over the frequency range of the translated components. The stage also includes peak-rectifier means, including the cathode and control electrode of tube 35 as well as a load resistor ii connected directly between the cathode and the control electrode, which is" substantiaily unresponsive to the gain control for stabilizing the translated modulation signal at a predetermined level thereof, specifically, for stabilizing I the translated modulation signal at the level of l which have been found to be particularly advan- Y of a given amplitude, the effective voltage swing oi the input signal, as applied to th input cir-' contrast ratio of the reproduced signal. Re-

tageous.

Condenser 44 microfarad 0.1 Resistor i1 -megohrns 2 Resistor 40 ohms 0-1.,000- Tube 35 Type 1852 Impedance of the load circuit of tube 35 ohms 5,000

the peaks oi the synchronizing components.

While applicant does not intend to be limited to any given circuit values for the embodiment of the invention illustrated, there follow values of circuit elements for a particular embodiment An. embodimento'i the invention utilizing the particular circuit values. given above has been found to provide relative gain-frequency characteristics substantially as shown in Fig. 3 for difierent adjustments of the tap on resistor 40, eflecpresent considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: p

l. A television signal-translating system comprising, a signal-translating channel adapted to have applied thereto an unstabilized television modulation signal having trace and retrace intervals and varying during trace intervals in accordance with video-frequency components having a wide range of frequencies and during retrace intervals in accordance with synchronizingsignal components, a video-frequency stage in said channel including a single vacuum-tube amplifier having an input circuit, means for adjusting the effective transconductance of said vacuum tube to control the gain of said amplifier over the frequency range of said components, and means comprised'within said stage and substantially unresponsive to said gain control for stabilizingthe translated modulation signal at the input circuit of said amplifier at a predetermined level thereof.

2. A television signal-translating system comprising, a signal-translating channel adapted to have applied thereto an unstabilized television modulation signal having trace and retrace in-' tervals and varying during trace intervals in accordance with video-frequency components having a wide range of frequencies and during retrace intervals in accordance with synchronizingsignal components, a video-frequency stage in said channel including a single vacuum-tube amplifier having an input circuit, means for adjusting the effective transconductance of said vacuum tube to control the gain of said amplifier over the frequency range of said components, and means comprised within said stage and substantially unresponsive to said gain control for stabilizing the translated modulation signal at the input circuit of said amplifierat the level of the peaks of said synchronizing components.

3. A television signal-translating system com- 5. A television signal-translating system comprising, a signal-translating channel adapted to have applied thereto an unstabilized television modulation signal having trace and retrace intervals and varying during trace intervals in acprising, a signal-translating channel adapted to have applied thereto an unstabilized television modulation signal having trace and retrace intervals and varying during trace intervals in accordance with video-frequency components having a wide range of frequencies and during retrace intervals in accordance with synchronizingsignal components, a video-frequency stage in said channel including a single degenerative vacuum-tube amplifier having an input circuit, means for adjusting the degeneration of said vacuum-tube amplifier to control the gain of said amplifier over the frequency range of said components, and means comprised within said stage and substantially unresponsive to said gain control for stabilizing the translated modulation si nal at the input circuit of said amplifier at a predetermined level thereof,

4, A television signal-translating system comprising, a signal-translating channel adapted to have applied thereto an unstabilized television modulation signal having trace and retrace intervals and varying during trace intervals in accordance with video-frequency components having a wide range of frequencies and during re-' trace intervals in accordance with synchronizing-signal components, a video-frequency stage in said channel including a. single vacuum-tube cordance with video-frequency components having a wide range of frequencies and during retrace intervals in accordance with synchronizing-signal components, an amplifier in said channel comprising a vacuum tube having a cathode, a control electrode, and an anode, means for adjusting the effective transconductance between said control electrode and said anode to control the gain of said amplifier over the frequency range of said components, and means including said control electrode and said cathode connected to operate as a peak rectifier and substantially unresponsive to said gain control for stabilizing the translated modulation signal at the input circuit of said vacuum tube at a predetermined level thereof. i

6. A television signal-translating system comprising, a signal-translating channel adapted to have applied thereto an unstabilized television modulation signal having trace and retrace intervals and varying during trace intervals in accordance with video-frequency components having a wide range of frequency and during retrace intervals in accordance with synchronizing-signal components, an amplifier in said channel comprising a vacuum tube having a cathode, a control electrode, and an anode, means for adjusting the effective transconductance between said control electrode and said anode to control the gain of said amplifier over the frequency range of said components, and peak-rectifier means including said control electrode, said cathode. and a load resistor connected directly between said control electrode and said cathode and substantially unresponsive to said gain control for stabilizing the translated modulation signal at the input circuit of said vacuum tube at a predetermined level thereof.

7. A television signal-translating system comprising, a signal-translating channel adapted to have applied thereto an unstabillzed television modulation signal having trace and retrace intervals and varying during trace intervals in accordance with video-frequency components having a wide range of frequencies and during retrace intervals in accordance with synchronizingsignal components, a vacuum-tube amplifier in said channel including a cathode, a control electrode, and an anode, means including an unbypassed cathode resistor for said vacuum tube ,75 8. A television signal receiver comprising a video-frequency translating channel adapted to V have applied thereto an unstabillzed television modulation signal having trace'and retrace intervals and varying during trace intervals in accordance with video-frequency components having a wide rangepf irequenciesandduring-retrace intervals in accordance with synchronizingsignal components, a video-frequency stage in said channel including a single vacuum tube amplifier having an input circuit, a contrast-ratio 10 control means for said receiver including means for adJusting the etlective transconductance of said vacuum tube to control the gain of said amplifier over the frequency range of said components. andm'eans comprised within said stages I "and substantially unresponsive to said contrast ratio control for stabilizing the translated modulation signal'atthe input circuit of said vacuum tube at a predetermined level thereof.

ARTHUR V. LOUGHREN. 

